ICS103 Programming in C
Lecture 5: Introduction to Functions
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Outline
 Introduction to Functions
 Predefined Functions and Code Reuse
 User-defined void Functions without Arguments
 Function Prototypes
 Function Definitions
 Placement of Functions in a Program
 Program Style
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Introduction to Functions
 So far, we have learnt how to use operators, +, -, *, / and %
to form simple arithmetic expressions.
 However, we are not yet able to write many other
mathematical expressions we are used to.
 For example, we cannot yet represent any of the following
expressions in C:
 C does not have operators for “square root”, “absolute
value”, sine, log, etc.
 Instead, C provides program units called functions to carry
out these and other mathematical operations.
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Introduction to Functions …
 A function can be thought of as a black box that takes one or more
input arguments and produces a single output value.
 For example, the following shows how to use the sqrt function that is
available in the standard math library:
y = sqrt (x);
 If x is 16, the function computes the square root of 16. The result, 4,
is then assigned to the variable y.
 The expression part of the assignment statement is called function call.
 Another example is: z = 5.7 + sqrt (w);
If w = 9, z is assigned 5.7 + 3, which is 8.7.
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Example
/* Performs three square root computations */
#include <stdio.h> /* definitions of printf, scanf */
#include <math.h> /* definition of sqrt */
int main(void) {
double first, second, /* input - two data values
*/
first_sqrt, /* output - square root of first input */
second_sqrt, /* output - square root of second input */
sum_sqrt; /* output - square root of sum
*/
printf("Enter a number> ");
scanf("%lf", &first);
first_sqrt = sqrt(first);
printf("Square root of the number is %.2f\n", first_sqrt);
printf("Enter a second number> ");
scanf("%lf", &second);
second_sqrt = sqrt(second);
printf("Square root of the second number is %.2f\n", second_sqrt);
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sum_sqrt = sqrt(first + second);
printf("Square root of the sum of the 2 numbers is %.2f\n",sum_sqrt);
return (0);
}
Predefined Functions and Code Reuse
 The primary goal of software engineering is to write error-free
code.
 Reusing code that has already been written & tested is one way to
achieve this. --- ”Why reinvent the wheel?”
 C promotes reuse by providing many predefined functions. e.g.
 Mathematical computations.
 Input/Output: e.g. printf, scanf
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Predefined Functions and Code Reuse …
 The next slide lists some commonly used mathematical functions
(Table 3.6 in the book)
 Appendix B gives a more extensive lists of standard library functions.
 In order to use a function, you must use #include with the
appropriate library.
 Example, to use function sqrt you must include math.h.
 If a functions is called with a numeric argument that is not of the
argument type listed, the argument value is converted to the required
type before it is used.
 Conversion of type int to type double cause no problems
 Conversion of type double to type int leads to the loss of any
fractional part.
 Make sure you look at documentation for the function so you use it
correctly.
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Some Mathematical Library Functions
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Function
Header File
Purpose
Arguments
Result
abs(x)
<stdlib.h>
Returns the absolute
value of its integer
argument x.
int
int
sin(x),cos(x),
tan(x)
<math.h>
Returns the sine, cosine,
or tangent of angle x.
double
(in radians)
double
log(x)
<math.h>
Returns the natural log of
x.
double (must be
positive)
double
Log10(x)
<math.h>
Returns base 10 log of x
Double (positive)
double
pow(x, y)
<math.h>
Returns xy
double, double
double
sqrt(x)
<math.h>
double (must be
positive)
double
x
Example
 We can use C functions pow and sqrt to compute the roots of a
quadratic equation in x of the form:
 If the discriminant (b2 – 4ac) is greater than zero, the two roots are
defined as:
 In C, these two roots are computed as:
/* compute two roots, root_1 and root_2, for disc > 0.0 */
disc = pow(b, 2) - 4 * a * c;
root_1 = (-b + sqrt(disc)) / (2 * a);
root_2 = (-b - sqrt(disc)) / (2 * a);
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Simple User-defined Functions
 An advantage of using predefined functions is that the programmer
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needs to be concerned only with what the function does but not how it
does it.
In complex software systems, this principle of separating what from the
how is an important aspect of managing the complexity of programs.
C provides a mechanism for the programmer to define his own functions
with the same advantages as the C’s library functions.
We now study the simplest type of user-defined functions – those that
display one or more lines of output.
These are useful for tasks such as displaying instructions to the user on
how to use a program.
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Example …
/* Performs three square root computations */
#include <stdio.h> /* definitions of printf, scanf */
#include <math.h> /* definition of sqrt */
void instruct (void); //displays user instruction
int main(void) {
double first, second, /* input - two data values
*/
first_sqrt, /* output - square root of first input */
second_sqrt, /* output - square root of second input */
sum_sqrt; /* output - square root of sum
*/
/* Display instrctions */
instruct();
printf("Enter a number> ");
scanf("%lf", &first);
first_sqrt = sqrt(first);
printf("Square root of the number is %.2f\n", first_sqrt);
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// continue next slide …
Example
// continue from previous slide
printf("Enter a second number> ");
scanf("%lf", &second);
second_sqrt = sqrt(second);
printf("Square root of the second number is %.2f\n", second_sqrt);
sum_sqrt = sqrt(first + second);
printf("Square root of the sum of the 2 numbers is %.2f\n",sum_sqrt);
return (0);
} // end of main function
/* displays user instructions */
void instruct(void) {
printf("This program demostrates the use of the \n");
printf("math library function sqrt (square root). \n");
printf("you will be asked to enter two numbers -- \n");
printf("the program will display the square root of \n");
printf("each number and the square root of their sum. \n\n");
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}
Function Prototypes
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Like other identifiers in C, a function must be declared before it can
be used in a program.
To do this, you can add a function prototype before main to
tell the compiler what functions you are planning to use.
A function prototype tells the C compiler:
1. The data type the function will return

2.
3.
The function name
Information about the arguments that the function expects.
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For example, the sqrt function returns a type of double.
The sqrt function expects a double argument.
So the function prototype for sqrt would be:
double sqrt(double);
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Function Prototypes : void Functions
 void instruct(void); is a void function
 Void function - does not return a value
 The function just does something without communicating anything back to its
caller.
 If the arguments are void as well, it means the function doesn’t take
any arguments.
 Now, we can understand what our main function means:
int main(void)
 This means that the function main takes no arguments, and returns
an int
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Function Definition
 The prototype tells the compiler what arguments the function takes
and what it returns, but not what it does.
 We define our own functions just like we do the main function
 Function Header – The same as the prototype, except it is not
ended by the symbol ;
 Function Body – A code block enclosed by {}, containing
variable declarations and executable statements.
 In the function body, we define what actually the function does
 In this case, we call printf 5 times to display user instructions.
 Because it is a void function, we can omit the return statement.
 Control returns to main after the instructions are displayed.
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Placement of Functions in a
program
 In general, we will declare all of our function prototypes at
the beginning (after #include or #define)
 This is followed by the main function
 After that, we define all of our functions.
 However, this is just a convention.
 As long as a function’s prototype appears before it is used, it
doesn’t matter where in the file it is defined.
 The order we define them in does not have any impact on
how they are executed
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Execution Order of Functions
 Execution order of functions is determined by the order of
execution of the function call statements.
 Because the prototypes for the function subprograms appear
before the main function, the compiler processes the
function prototypes before it translates the main function.
 The information in each prototype enables the compiler to
correctly translate a call to that function.
 After compiling the main function, the compiler translates
each function subprogram.
 At the end of a function, control always returns to the point
where it was called.
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Flow of Control Between the main
Function and a Function Subprogram
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Program Style
 Each function should begin with a comment that
describes its purpose.
 If the function subprograms were more complex, we
would include comments on each major algorithm step
just as we do in function main.
 It is recommended that you put prototypes for all
functions at the top, and then define them all after
main.
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